Certain light transmitting structures have been proposed for use in responding to input incident radiation and in redirecting such radiation toward a generally defined and relatively confined region. For example, holographic window structures have been proposed for responding to sunlight radiation and for redirecting such radiation in a selected direction toward a desired interior region of a room for illumination thereof. One of the problems with such structures has been that, as the angle of incidence of the incoming sunlight radiation changes, either in azimuth or elevation, or both, the direction of the redirected illuminating radiation changes within the room. It is desirable to provide a window structure which would experience only small changes in the redirected light, even though the angle of incidence of the incoming radiation changes over a relatively wide range.
The use of holograms for such purpose has been suggested, both in the form of structures known to those in the art as "thin" holograms and "volume" holograms. While thin holograms tend to accept and respond to incoming radiation over a relatively wide range of incidence angles, it has been found that, as the angle of incidence changes, the direction of the output radiation changes markedly. In addition thin holograms do not have as high an efficiency as volume holograms, that is, a considerable fraction of the incident light (of the order of 50% typically) continues undiffracted through the hologram. On the other hand, while volume holograms tend to diffract incident incoming radiation so that it remains within relatively limited confines and with high efficiency, the hologram responds to incoming radiation only over a relatively narrow range of incidence angles.
It has been further suggested that, in order to overcome the above problems, a diffracting structure be used which is formed by utilizing a plurality of layers, or laminations, of structural elements each having diffraction characteristics selected so as to control the radiation transmitted through the structure in a selected manner. Such a concept is disclosed, for example, in the copending U.S. patent application, Ser. No. 738,300, filed on May 28, 1985 by Hendrik J. Gerritsen et al.
A typical three-layer embodiment as disclosed therein includes two outer diffracting elements each having, for example, a selected topographical diffraction relief pattern formed on at least one surface thereof, and an intermediate element having a selected topographical diffraction relief pattern formed on at least one surface thereof positioned between them. The intermediate element has an index of refraction which is selected to be significantly different from the index of refraction of either of the two outer structural elements. The use of a plurality of such layered, or laminated, embodiments in an overall structure is found to respond to incoming radiation over a relatively wide range of input incidence angles and over a relatively wide band of wavelengths and, when used as a window structure, directs such radiation toward a selected region which remains within relatively limited confines for all visible wavelengths.
However, while such structures are satisfactory in many applications, it is found that the range of output angles over which the desired operation can be achieved may not be narrow enough in some applications, i.e., as the range of incidence angles increases, the region to which the exiting radiation can be confined also increases. Hence, a trade off must be made between the range of incident angles to which the structure can respond and the range of exit angles that results, i.e., the wider the range of incident angles of acceptance the wider the range of exit angles. Moreover, as the number of layers of material needed to achieve a wider range of incidence angles is increased, the more complex the structure becomes and the more difficult it is to fabricate. Furthermore, reflections of the incident and the diffracted radiation occur at the interfaces between layers of different refractive indices reducing the efficiency.